1. Field of the Invention
The present invention relates to a semiconductor light-receiving device used for optical fiber communication or the like.
2. Background Art
Structures of semiconductor light-receiving devices can be roughly divided into a surface illuminated type in which light is incident from a surface side of an epitaxial growth layer on a semiconductor substrate, an edge illuminated type in which light is incident from a side face of the epitaxial growth layer, and a back-side illuminated type in which light is incident from a semiconductor substrate side.
Generally, semiconductor light-receiving devices can obtain light receiving sensitivity only in a region of PN junction which is built in the structures, and the greater the size, the easier the alignment of light, excelling in mountability. On the other hand, when the PN junction increases in size, the device capacity increases and its time constant increases, which is disadvantageous in terms of high-speed response.
Among the above-described structures of light-receiving devices, the back-side illuminated type structure can easily reduce a parasitic capacitance and can thereby increase the size of PN junction, making it easier to achieve both ease of mounting and speed enhancement by a reduction in the device capacity. The back-side illuminated type structure is therefore a common structure particularly for high-speed communication of 10 Gbps or higher.
Semiconductor light-receiving devices are often manufactured by laminating an undoped light absorption layer and a p-type conductive region in that order on an n-type conductive layer formed on an n-type substrate or semi-insulating substrate. In the semiconductor light-receiving devices, many carriers are generated at a position of the light absorption layer close to the light incident surface. Therefore, in common back-side illuminated type light-receiving devices, many carriers are generated on a side of the light absorption layer close to the substrate. In this case, positive holes having a low drift speed move up to the p-type conductive region, substantially drifting throughout the thickness of the light absorption layer, and therefore the movement of carriers takes time and the high-speed response property deteriorates.
In order to solve this problem, a back-side illuminated type light-receiving device is conceived in which a p-type conductive layer is formed on a semi-insulating substrate, and an undoped light absorption layer and an n-type conductive layer are formed thereon (e.g., see Japanese Patent Publication No. 5327892 and Japanese Patent Publication No. 5497686). In this structure, since many positive holes generated on a side of the light absorption layer close to the substrate drift toward the substrate side, the moving distance becomes shorter and the high-speed response property improves. An example of APD is also conceived which has a floating guard ring not connected to any electrode (e.g., see Japanese Patent Application Laid-Open No. 2008-021725, Japanese Patent Application Laid-Open No. 10-284754 and Japanese Patent Application Laid-Open No. 2012-054478).